1. Field of the Invention
The present invention relates to an active-type display wherein luminescent elements, e.g. electroluminescent (EL) elements or fluorescent elements are driven with thin film transistors (TFTs).
2. Description of Related Art
Organic EL elements are self-emitting displays. The organic EL element does not require any backlight indispensable for liquid-crystal displays and is suitable for thinning the display structure. Since the viewing angle of the organic EL element is not limited, the organic EL element has been highly expected to be put to practical use as the next-generation display.
In order to form an organic EL panel 1, as shown in FIG. 1, an anode electrode 3 formed of e.g. an ITO transparent electrode is formed on a transparent glass substrate 2. A hole-transport layer 5 of MTDATA (4,4′-bis(3-methylphenylphenylamino)biphenyl), an emissive layer 6 of TPD (4,4′,4″-tris(3-methylphenylphenylamino)triphenylanine) and Rubrene, and an electron-transport layer 7 of Alq3 are sequentially deposited between the anode electrode 3 and a cathode electrode 4 of MgIn alloy. When holes injected from the anode electrode 3 and electrons injected from the cathode electrode 4 are recombined within the emissive layer 6, light is emitted out from the transparent anode.
The display which drives the organic EL elements is classified into two types: a passive type using a passive matrix and an active type using TFTS. The schematic configuration of the passive type display is shown in FIG. 2.
A pair of electrodes in an EL element includes an anode electrode 3 in column and a cathode electrode 4 in row. Anode electrodes and cathode electrodes are arranged in a matrix. A row driver 8 supplies scanning signals ROW1, ROW2, ROW3, . . . to the cathode electrodes 4 respectively to select one of plural rows. The column driver 9 receives gray scale (tone) data representing a display gray scale of each pixel and then outputs pulse signals with a pulse width corresponding to the gray scale data, as column drive signals COL1, COL2, COL3, . . . , COLm. Thus, the EL elements emit light, corresponding to pixels PX1, PX2, PX3, . . . , PXm to which scanning signals and column drive signals are supplied.
Usually, in order to apply a predetermined positive voltage to the anode electrodes and to apply the ground potential or negative voltage to the cathode electrodes, a signal input substrate 10 such as TAB or FPC is connected to the organic EL panel 1. As shown in FIG. 1, copper connection terminals 11 are formed on the back surface of the signal input substrate 10 to supply predetermined voltages to the corresponding cathode electrodes. Conventionally, to connect the connection terminal 11 to the corresponding cathode electrode 4, a cathode material is extended to the connection terminal without any change.
In the passive-type display, electrons from EL elements corresponding to all pixels in the same column sink into the cathode electrodes. In the active-type display using TFT transistors, a sole cathode electrode is formed as a common electrode over the whole of the panel display area. All currents, which are emitted from EL elements corresponding to all pixels within a display area, are sank into the cathode. This current value becomes very large. Hence, like the conventional passive-type display, if the cathode material, in the active-type display, is extended from the cathode electrode to the panel terminal, as the connection conductor to the connection terminal of the signal input substrate, the connection conductor is narrowed significantly, compared with the cathode electrode. As a result, a large current from the cathode electrode becomes concentrated in the connection conductor. The cathode electrode made of Mg alloy has a relatively large resistance value. The large resistance component of the connection conductor causes a large voltage drop, thus decreasing the intensity of an EL element. This problem becomes more serious as the display screen is enlarged.